molecular reaction dynamics. collision theory of kinetics with few exceptions, the reaction rate...
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Molecular Reaction Dynamics
Collision Theory of KineticsWith few exceptions, the reaction rate
increases with increasing temperature temperature
If we assume that a chemical reaction takes place due to collisions between reactant molecules
i.e. rate number of collisions / unit time
A2 + B2 product rate = k2[A2][B2]
“collision of A2 and B2 causes a reaction”
Rate constant vs. T
The Reaction Profile
The Activation EnergyThe minimum amount
of energy need for initiation of a chemical reaction is the activation energy (Ea).
Colliding reactant molecules possess kinetic energy > the activation energy or Ea.
The Activated ComplexActivated Complex
the species temporarily formed by the reactant molecules as a result of collisions
A small fraction of molecules usually have the required kinetic energy to get to the transition state the concentration of the activated complex is
extremely small.
The Arrhenius EquationArrhenius showed how the rate constant
depended on temperature.
/RT A - E k alnln
The frequency factor
The activation energy
The Arrhenius Equation
Estimating Rate ConstantsWe can use collision theory to provide a
basis estimating the rate constant.What if all collisions gave products?
AB
2d Zdt
AdN
The Energy RequirementSince we know only a fraction of collisions
will give products
RTE
ABAB2d
a
eZfZdt
AdN
RTaE
ef
Converting to Molar Amounts RT
E
ABAB2d
a
eZfZdt
AdN
V
AnNVAN
AN 2Avo22d
RTE
Avo
AB2a
eNZ
dtAd
2d2d2
AB
21
ABAB BNANd
RT8Z
22RT
E
Avo2
AB
21
AB
2 BAeNdRT8
dtAd a
RTE
Avo2
AB
21
AB2
a
eNdRT8
k
The Geometrical RequirementThere are many collisions of sufficient energy
that do not yield productsDefine the steric factor P to account for local
properties of the moleculeOrientations during collisionsSubstituent effects
Effective Collisions
Ineffective Collisions
The Steric Factor and k2
RTE
Avo2
AB
21
AB2
a
eNdRT8
Pk
Note values of P can be quite small Usually in the range < 0.001
Limitations of Collision TheorySimple Collision Theory
Best suited for studying reactions between simple species (atoms, diatomic molecules).
P factor indicates how reactants collide becomes very important when the species get bigger.
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